Conventionally, many drawbacks of display devices in terms of color reproducibility have been pointed out. In particular, it has been pointed out that liquid crystal display devices have the following two drawbacks.
Many liquid crystal display devices allow light transmittance by using the birefringent characteristic of liquid crystal. However, liquid crystal of different pixels (pixels for R, G, and B) show different transmittance with respect to the same voltage. Therefore, even if the same color (e.g. white (R=G=B)) is displayed, the hue could be different, depending on gradation.
An effective measure to solve this problem is setting an independent γ curve with respect to each of R, G, and B, either in an analog or digital way. For example, Patent Publication 1 (Japanese Publication for Laid-Open Patent Application, Tokukai 2002-258813 (publication date: Sep. 11, 2002)) discloses such a technique for independently correcting each of R, G, and B.
In shutter-type liquid crystal display devices, light of each color leaks regardless of display gradation. Especially, if the display gradation decreases, color purity (color saturation) decreases due to the light leakage. Moreover, since luminance efficiency is regarded as an important factor for many liquid crystal display devices, spectrum characteristics of backlight and color filters must be broad even if contrast is sufficient. Under such circumstance, the color saturation decreases as the luminance decreases.
An effective technique for improving the color purity is emphasizing the color saturation by increasing color saturation of such color that has relatively high color saturation, and decreasing color saturation of such color that has relatively low color saturation. For example, Patent Publication 2 (Japanese Publication for Laid-Open Patent Application, Tokukai 2003-52050 (publication date: Feb. 21, 2003)) discloses such a technique for correcting the color saturation.
In addition, the problem of crosstalk, which is caused by the coupling of adjacent pixels through parasitic capacitance, has been pointed out as a problem unique to TFT-LCDs. If there is an insulating film between a transparent electrode and a source line, parasitic capacitance is formed there. Likewise, parasitic capacitance is formed between a gate line and the transparent electrode, and between a source line and a common electrode. Due to the influence of the parasitic capacitances and the capacitance of the liquid crystal itself, the potential of the display pixels could be different from desired voltages, when the gate is OFF. As a result, the display gradation could be different from a desired gradation. For example, Patent Publication 3 (Japanese Publication for Laid-Open Patent Application, Tokukaihei 5-203994 (publication date: Aug. 13, 1993)) discloses a technique for reducing the parasitic capacitances as a means of solving the problem of crosstalk. However, this technique is still insufficient to reduce the crosstalk.
Incidentally, although these prior arts are effective in order to adjust the color reproducibility of a panel as a whole or of each display pixel, these prior arts cannot respond to a situation where reproduced colors change in accordance with display patterns generated by a display device.
To a display pixel connected to a TFT, a desired voltage is applied at the moment the gate is high. On the other hand, when the gate is low, the display pixel is connected to many peripheral electric circuits through parasitic capacitances. Since many of these peripheral electric circuits are related to panel design, the driving voltage can be set in advance while considering the parasitic capacitance formed between the display pixel and the peripheral electric circuits. Thus, the crosstalk caused by the parasitic capacitances formed between the display pixel and the peripheral electric circuits can be compensated in advance. However, since the potentials of source lines for driving other display pixels cannot be determined in advance, the crosstalk caused due to the other source lines cannot be compensated in advance.
A liquid crystal display device shown in FIG. 15(a) is provided with source lines Si (i: integer) and gate lines Gj (j: integer) arranged to be orthogonal. At each intersection of a source line and a gate line, a display pixel 100 and a switching element 200 are provided. Among display pixels 100, each display pixel (A) is provided with parasitic capacitances Csda, Csdb, Cgd, and Ccs. A display pixel (B) is a display pixel adjacent to a display pixel (A) in the direction along which the gate lines are provided.
Details of the parasitic capacitances are as follows:                Parasitic capacitance Csda: parasitic capacitance formed between a display pixel (A) and the source line S2, which drives the display pixels (A);        Parasitic capacitance Csdb: parasitic capacitance formed between a display pixel (A) and the source line S3, which drives the display pixels (B);        Parasitic capacitance Cgd: parasitic capacitance formed between a display pixel (A) and the gate line G2, which drives the display pixel (A); and        Parasitic capacitance Ccs: parasitic capacitance formed between a common electrode line and a display pixel (A).        
The capacitance of a display pixel (A) itself is Cp. The voltages applied to the gate lines change as shown in FIG. 15(b). While a display pixel (A) displays G, a display pixel (B) displays R or B. If the display gradation of the display pixel (A) is LA, and the display gradation of the display pixel (B) is LB, LA≠LB.
In this case, at the time the gate is high, if a drain voltage +V(A) is applied to the liquid crystal part of the display pixel (A), a drain voltage −V(B) is applied to the liquid crystal part of the display pixel (B). When the next gate line turns ON, −V(A) is applied to the source line for driving the display pixel (A), and +V(B) is applied to the source line for driving the display pixel (B).
In reality, however, a drain voltage is not applied directly to the display pixel (A). Instead, a drain voltage changed by the influence of the parasitic capacitances is applied to the display pixel (A). Specifically, an effective value Va of a voltage applied to the display pixel (A) is represented byVa=V(A)+(Csda×V(A)+Cgd×Vg+Csdb×V(B)+Ccs×Vc)/Cp where Vg is a voltage applied to the gate line, and Vc is a voltage applied to an opposed electrode.
Thus, the voltage applied to the display pixel (A) is different from the desired drain voltage (A).
The parasitic capacitances Csda, Cgd, and Ccs, which are formed in the vicinity of the display pixel (A), can be estimated at the design stage. Therefore, the drain voltage can be set appropriately by considering values of the parasitic capacitances. This means that the parasitic capacitances do not have much influence on the display gradation of the display pixel (A).
However, the foregoing formula for calculating the effective voltage Va includes the parasitic capacitance Csdb and the drain voltage V(B). This means that the voltage Va is influenced by the source line connected to the display pixel (B). Therefore, depending on the display gradation of the display pixel (B), color crosstalk is caused (that is, the gradation of the display pixel (A) changes).
For example, when V(A)=±2.59V and V(B)=±1.21V, the voltage supplied to the display pixel (A) is ±2.45V. Thus, it is found that color balance changes.
Even if the parasitic capacitances are reduced at the design stage as disclosed in Patent Publication 3, the amount of crosstalk is only reduced. The color crosstalk cannot be eliminated completely. Therefore, the potential actually applied to the display pixel changes in accordance with the display pattern of the display device as a whole. As a result, the display pixel cannot reproduce desired luminance.
In theory, it is possible to compensate the crosstalk by providing new members, such as shield electrodes or wires. However, if new members are provided to the display device, production cost of the display device increases.